The present invention relates to a method for the production of polyamide 6 from polyamide waste and to an apparatus for the carrying out of this method.
In the production and processing of polyamide 6 pellets, polyamide 6 waste is produced which is to be further processed in order to improve the economy of the material.
Thus, polyamide 6 waste, if necessary comminuted, is melted down in an extruder and converted into a pelletizable strand. This process is frequently employed for the production of injection-molded pellets since the demands made on these pellets with respect to uniform molecular weight distribution and viscosity are not as great as on pellets which are to be worked into polyamide filament or film.
Furthermore, it is known to depolymerize polyamide waste down to the initial raw material, caprolactam. This method has the disadvantage that it is very detrimental to the environment and very expensive since phosphoric acid is used for the depolymerization and the lactam recovered must be repeatedly distilled to obtain a lactam of high purity. Furthermore, residues remain which must be disposed of.
From East German Patent 264,119, a method for the partial depolymerization of high-molecular polyamide 6 to caprolactam is known in which the caprolactam used as solvent and depolymerization agent is itself simultaneously polymerized, there being obtained therefrom a copolymer having a uniform, ordinary molecular weight distribution. Laboratory experiments have shown that while caprolactam is a good solvent for polyamide 6, it does not act as a depolymerization agent, which is also not possible from a standpoint of the chemism of the course of the polymerization. Only a solution mixture of high-molecular polyamide and lactam is produced. With a product temperature of about 270.degree. C., a polyaddition reaction takes place, i.e. lactam adds onto the non-depolymerized, highly viscous polyamide molecules insofar as the latter are capable of reaction at all. In addition to this, high-molecular polyamide particles which are strongly crosslinked beyond the normal extent, so-called gel particles, are present in the resultant polymer; they are produced upon the cooling of the polyamide waste in the atmosphere of the air, cannot be broken down by the method described in East German Patent 264,119, and thus accumulate in the polyamide.